1. Field of the Invention
The present invention relates to tubular connections of the kind commonly used in the oil industry. In particular, the tubular connection of the present invention comprises a stabilized center-shoulder seal, wherein stabilization is provided by specialized thread constructions adjacent the center-shoulder seal. The invention also pertains to a specialized center-shoulder seal design.
2. Background Art
The present invention is applicable broadly for use in joining pipes, couplings and the like, and has particularly been developed for use in oil industry tubular connections, since these connections experience more severe and demanding conditions than those observed for other industries and applications. In particular, the trend in the oilfield is to minimize pipe diameter and to conserve borehole diameter. The smaller the diameter of the borehole for the well, the greater the conservation of energy and reduction of drilling costs. However, in utilizing smaller diameter pipe, it is important that the connections between adjacent tubular pipes be transparent in geometry and performance characteristics, i.e. the geometry and performance characteristics of the connection be substantially the same as that of the tubular pipe itself. Preferably, the pipe string should behave as if the pipe connections were not there. The ideal connection would have the same geometry as the pipe, such as the same inside and outside diameters, and would have the same performance characteristics such as tension rating, compression rating, internal and external pressure ratings, torsion resistance, and bending resistance, for example.
Two types of oilfield connections, namely flush joints and slim line connections, have been utilized to conserve hole diameter. The outer diameter of a flush joint connection is generally the same as the outside diameter of the tubular pipe. The outside diameter of a slim line connection is generally 2 to 3.5% greater than that of the pipe. Although the objective of such prior art pipe has been to achieve total geometric and performance characteristic transparency, such has not been possible. Often, the performance characteristics of prior art pipe have been sacrificed to achieve greater geometric transparency.
Various standards have been set for evaluating the efficiency of a pipe connection. One such efficiency is determined by the load capacity of the pipe and pipe connection in the axial direction. This tension efficiency is calculated by comparing the fracture strength of the connection with the fracture strength of the tubular pipe. Most prior art flush joint and slim line connections achieve an efficiency in the range of 65% to 75%. This efficiency, of course, varies with the type of connection, the pipe diameter and thickness, and the method of manufacture of the connection.
Prior art connections include the basic elements of threads, shoulders, and seals. The following prior art patents describe various features which have been used to produce flush and slim-line connections.
U.S. Pat. No. 1,927,656, issued Sep. 19, 1933 to G. M. Eaton et at., describes a pipe joint for fastening together the consecutive pipe lengths of a pipe string in a manner to obtain strength at the joint without increasing the wall thickness (or with a minimum increase thereof) in the portion of the pipe in which the screw threads are cut. One of the two pipes to be joined is machined on the outside surface of the pin and the other is machined on the inside surface of the box so the two pipes can be screwed together to provide what would today be called a slim line connection. The threaded portion of each pipe is machined on a taper with the depth of the turns of the threads being the greatest at the center of the threaded portion and decreasing gradually in both directions to vanishing points at the ends of the threaded portions.
U.S. Pat. No. 2,062,407, issued Dec. 1, 1936 to G. M. Eaton et al., discloses a preferred form of pipe joint wherein the height of the threads in the threaded portion of the joint is at a maximum in the central area of the threaded portion of the joint and decreases gradually in both directions to vanishing points, somewhat similar to the joint described above. However, the thread design differs in that the vanishing of the threads corresponds with the intersection of thread fabrication lines which are not only based on a taper relative to the pipe longitudinal axis but also a line which is cylindrical or parallel to the pipe longitudinal axis. Specifically, the root of the thread at the pipe exterior edge position of the pin is located along a line parallel with the pipe longitudinal axis while the crest of the thread at the same location is located along a line at a constant taper with the pipe longitudinal axis. The root of the thread at a position more interior from the edge of the pipe is located along a line at a constant taper with the pipe longitudinal axis, while the crest of the thread at the same location is located on a line of parallel taper to the root for a distance. Finally, at the most interior thread location on the pin, the thread root continues on a taper with the pipe longitudinal axis while the crest of the thread is constructed along a line parallel with the pipe longitudinal axis. Thus, at both the exterior and interior edges of the threaded connection, the thread disappears where thread root and thread crest intersect. The thread construction of the box is complimentary to that of the pin so the two threaded members can be interengaged.
U.S. Pat. No. 3,109,672, issued Nov. 5, 1963 to William F. Franz describes a threaded pipe joint comprising a pipe member having a cylindrical outer surface and a tapered buttress thread at the end thereof vanishing along the outer cylindrical surface, providing a length of fully formed and a length of vanishing threads. The complementary coupling member having thread machined on the internal surface thereof exhibits a full height tapered thread throughout the entire length thereof. The complementary threads on each member have following flanks in bearing relationship and substantially normal to the longitudinal axis of the joint and leading flanks in bearing relationship, which leading flanks have a larger flank angle than the following flanks. Crests and roots are truncated to provide flat crests and roots which are parallel to the longitudinal axis of the joint. Crest truncations of fully formed threads exceed root truncations to provide a predetermined amount of void between the crests and roots of the complementary threads throughout the length of the joint when the pipe and coupling are in hand tight engagement. After power make-up, the crests of the coupling threads engage the roots of the vanishing pipe threads, but voids still remain between crests and roots of the fully formed threads, thus preventing the development of deleterious hoop stresses at the end of the coupling during power make-up.
U.S. Pat. No. 3,989,284, issued Nov. 2, 1976 to Thomas L. Blose (see also Re. 30,647, issued Jun. 16, 1981) describes a tubular connection (pipe joint) designed to produce hoop tension in the pin member and hoop compression in the box member, the connection exhibits threads having a dove-tail interfit. To achieve the dove-tail interfit, the angles formed between the thread flanks and the root wall of the thread are less than about 85 degrees (i.e. the flank angles are negative angles of at least 5 degrees with a line perpendicular to the baseline at the root of the thread). In another embodiment of the invention, the threads exhibit a negative angle of at least 5 degrees on the load flank of the pin member, with the stab flank being at 90 degrees with (perpendicular to) the baseline of the root of the thread. The pin thread stab flank is the thread flank which is on the leading edge as the pin is telescoped (stabbed) into the box. The load flank is the trailing flank of the thread. In all cases, the threads are designed with clearances such that there is a dove-tail interfit between pin and box drawing the pin wall outwardly and the box wall inwardly to produce hoop tension in the pin member and hoop compression in the box member in a made up connection, i.e. there is a wedging interfit of threads in the made up joint.
Each thread member is disclosed in the '284 Blose patent as having progressively changing axial width along substantially the entire helical length thereof. Although the preferred embodiment appears to have the threads constructed on a line parallel to the pipe longitudinal axis, the possibility is disclosed of having the threads constructed on a line having constant taper from the pipe longitudinal axis. Also described is the use of an inner shoulder seal between threaded sections of the connection. In one embodiment, the inner shoulder seal is placed between two thread sections which fall on two different lines, each parallel to the pipe longitudinal axis, creating a multiple-step thread with an inner shoulder seal. The inner shoulder seal is shown and described as having a major mating (sealing) surface which is parallel to the pipe longitudinal axis or a major mating (sealing) surface which is at an axial taper.
U.S. Pat. No. 4,009,893, issued Mar. 1, 1977 to Schatton et al., discloses box and pin members having two axially spaced thread sections being separated by a radial step zone which includes, for one member, e.g. the pin member, an annulus having an axial undercut, a frusto-conical end face and a radially outward bulging peripheral surface, which sealingly engages a slightly contoured surface of the box member in the step zone, while the end face of the annulus bears against a complementary, slightly contoured face in the step zone of the box member. These stop faces supplement complementary thread stop faces respectively at the end of the pin member and box member. It is advised that a flat annulus having good sliding properties, such as teflon, be interposed between the pin member annulus end face and the complementary bearing face of the box member. In one embodiment, there is a space between the pin member annulus end face and the complementary bearing face of the box member, and a sealing sleeve is interposed to fill this space and the undercut space of the pin member annulus.
U.S. Pat. No. 4,161,332, issued Jul. 17, 1979 to Thomas L. Blose describes a pipe joint including pin and box members having interengaged two-step threads and interengaged annular shoulders. One of the annular shoulders of the connection is located axially between the first and second pair of threads. This shoulder is constructed so that the pin member stab flank of the shoulder is at a negative angle to the pipe longitudinal axis (preferably at about 5 degrees) to provide a semi-dovetail interfit of the shoulder on makeup of the joint. The pin member threads in general exhibit a negative angle load flank (preferably about 15 degrees) and a negative stab flank angle (typically about 30 degrees), to provide a semi-dovetail interfit with corresponding box threads; the pin member stab flank is typically not engaged, there being about 0.020 inches clearance with the corresponding box threads on normal make-up.
U.S. Pat. No. 4,398,756, issued Aug. 16, 1983 to Duret et at., describes a cylindro-conical pipe joint wherein approximately 15% to 25% of the joint is threaded upon a continuous construction line which is at a taper to the longitudinal axis of the pipe, with the remaining approximately 75% to 85% of the threaded portion of the joint being threaded upon a continuous construction line parallel to the longitudinal axis of the pipe. The position of the threading which is on a taper to the pipe longitudinal axis is that which forms the entry to the connection when the pin and box are initially being telescoped together. Thus, clearance is provided by the taper of the box entry to permit ease in initial telescoping of the pin into the box, prior to actual engagement of the threads.
U.S. Pat. No. 4,591,195 issued May 27, 1986 to Chelette et al. discloses a center shoulder seal located between two threaded portions of a pipe joint, wherein the threaded portions are stepped threads constructed on a line parallel to the longitudinal axis of the pipe. The center shoulder seal is fabricated so the pin portion of the seal contains an undercut groove which can be used in combination with a sealing ring of rigid plastic or rubber material to provide a seal capable of retaining hot gases or liquids.
U.S. Pat. No. 4,671,544, issued Jun. 9, 1987 to Donald J. Ortloff discloses a threaded pipe connection having two portions of tapered threads with tapered sealing surfaces located between the threaded portions. The tapered sealing surfaces engage a resilient seal member located in a groove about midway between the ends of one of the seal member surfaces; when the joint is made up, the sealing surfaces form independent metal-to-metal seals on both sides of the resilient seal member. In a preferred embodiment, the threaded portions are in the form of two steps, one on each side of the center sealing surface described above; the threads are wedge-shaped threads that increase in width progressively in one direction within each step of threads.
U.S. Pat. No. 4,676,529, issued Jun. 30, 1987 to Patrick E. McDonald describes a pipe joint wherein the pin and box members have two stepped portions of threads, each constructed on a line parallel with the longitudinal axis of the pipe, and having metal-to-metal sealing surfaces located between the threaded portions. The sealing surfaces provide three engaging shoulders between the pin and box; the two sets of outer shoulders have space between them at the time the set of inner shoulders is in contact. Between the set of inner shoulders and each set of outer shoulders, there is a groove; in one location the groove is in the box portion of the connection and in the other location the groove is in the pin portion of the connection. When the connection make up torque is beyond the yield point of the metal of the set of inner shoulders, the metal extrudes into the grooves in the box and pin members to form interlocking surfaces which hold the box and pin from relative rotation.
U.S. Pat. No. 4,703,954, issued Nov. 3, 1987 to Ortloff et al. describes a threaded pipe connection comprising a box having tapered internal threads and a pin having tapered external threads, wherein the threads are dovetail shaped in cross section (wedge shaped thread) and increase in width in one direction on the box and in the other direction on the pin. This design is improved over that described in U.S. Pat. No. 3,989,284 (Re. 30,647) in that high stress concentrations are avoided in the box between the first thread and the end of the box by making the length of the box between the first thread and the end of the box equal to or greater than 5% of the pipe diameter and by providing clearance between the crest of the first full thread on the box and the root of the last thread on the pin and between the crest of the last full thread on the pin and the root of the first full thread on the box.
U.S. Pat. No. 4,753,460, issued Jun. 28, 1988 to Lawrence Y. Tung, discloses a tubular connection having a small diameter thread set and a large diameter thread set, the small thread set having a lesser number of threads than the large set. A primary torque shoulder is located between the thread sets and a secondary torque shoulder is located behind the large thread set. In the preferred embodiment, the initial partial thread on the pin of both sets and the last partial thread of the small set on the box has a cylindrical root contour (the root contour is constructed on a line parallel with the longitudinal axis of the pipe), whereas the remainder of threads exhibit a tapered root contour. The longitudinal taper angle of the pin thread sets are dissimilar to the box thread sets, there being a maximum separation of the sets near the center of the connection. At the center, a seal is provided; the last threads adjacent to the center shoulder on the threaded sets are hooked (the angle of the pin load flank is negative with respect to the pipe longitudinal axis). In all embodiments described, the threads exhibit a negative load flank and a positive stab flank. The center shoulder area of the pin exhibits two flanks, one of which is positive (near the small diameter step of threads) and one of which is negative (near the center of the shoulder and about midway between the two steps of threads).
U.S. Pat. No. 4,917,409, issued Apr. 17, 1990 to Doyle E. Reeves describes a threaded connection employing a tapered, wedge shaped thread. The threads increase in width in one direction on the box and in the other direction on the pin. The improvement over the prior art appears to be in the ability of the thread design to be used with pipe thread lubricants; to accommodate such lubricants, the thread is designed "with the stab flanks and the roots and crests being designed to move into sealing engagement before both the load flanks and the stab flanks move into sealing engagement after which the flanks will move together and form seals to complete the sealing of the connection as the connection is fully made up" (in the power tight position). This is accomplished by increasing the axial thread width at the widest portion in the thread to approximately four times the thread width at the narrowest portion of the thread, and by eliminating radial clearance when the joint is made up hand tight, i.e. the thread roots and thread crests of the corresponding box and pin threads come into interference contact prior to the connection being placed in the power tight position.
The present invention overcomes the deficiencies of the prior art connections and particularly achieves greater performance characteristic transparency without sacrificing geometric transparency.